(1) Field of the Invention
This invention relates generally to digital-to-analog converters and relates more specifically to logarithmic digital-to-analog converters.
(2) Description of the Prior Art
Logarithmic Digital-to-Analog converters (DACs) are required in a number of applications such as biasing LEDs.
Main disadvantage of prior art solutions for logarithmic DACs is the way to implement the linear to exponential conversion. Each current step value has to be calculated using a mathematical formula and it is implemented by adjusting the linear steps based on mathematical calculation. Especially higher resolution DACs require very accurate matching and therefore a large chip area. The complexity of prior art solutions and the chip-area occupied thereof increases exponentially as the resolution increases.
It is a challenge for the designers of logarithmic DACs to achieve simple solutions, which are easy to be implemented.
There are known patents or patent publications dealing with startup circuits for logarithmic DACs.
U.S. Patent Publication (US 2008/0204291 to Christ) proposes embodiments of a digital-to-analog converter (DAC) with a logarithmic response and methods for converting digital signals to analog. Other embodiments are described and claimed. In some embodiments, the DAC includes a wedge-shaped resistive array having a plurality of linearly spaced contact nodes and a switching array to selectively couple one of the contact nodes with an analog output based on a control signal. Each of the contact nodes may provide a corresponding reference voltage that varies logarithmically with respect to the linearly spaced contact nodes.
U.S. Patent (U.S. Pat. No. 6,208,278 to Toosky) discloses a digital to analog conversion. According to an embodiment of the present invention, a logarithmic transfer characteristic may be produced by subtracting a fraction of the output current to a reference current in a recursive equation. A reference current enters a circuit and flows through a transistor.
U.S. Patent (U.S. Pat. No. 7,078,668 to Yamazaki et al.) describes a photoelectric conversion apparatus including a photoelectric conversion element and a logarithmic conversion unit for converting a signal from the photoelectric conversion element to a logarithmically compressed voltage by means of a diode characteristic of p-n junction.